1. Technical Field of the Invention
The present invention relates to dielectrics. More particularly, it relates to a process for producing dielectrics which are superior in insulation characteristics, low in dielectric loss and can be fired at low temperatures, and a process for producing fine single crystal powders, especially perovskite type fine single crystal powders having a uniform particle size.
The dielectrics obtained by the present invention can be applied to dielectric devices such as thin film capacitors, capacitors for DRAM, multilayer capacitors and dielectric pastes and piezoelectric parts such as actuators, resonators, filters and ultrasonic motors. The fine single crystal powders can be applied to piezoelectric parts such as actuators and pressure-sensitive sensors as piezoelectric composites and furthermore, can be applied to phosphor materials by doping of rare earth ions.
2. Prior Art
Hitherto, dielectrics such as barium titanate and lead titanate have been utilized as ceramic capacitors and filters thanks to their dielectric characteristics and piezoelectricity.
These ceramics are generally used in the multilayer form so that their capacitance can be increased. The multilayer capacitors are produced by mixing dielectric powders of 0.5-5.mu. obtained by solid state reaction or precipitation from a solution with a binder and a solvent to prepare a slurry, molding the slurry to a thin sheet by a tape casting method, stacking ten to several tens of the resulting thin sheets and firing them at 1200.degree.-1300.degree. C. In this case, however, a high firing temperature is necessary and metals relatively inexpensive and low in resistance such as silver and copper cannot be used as electrodes. In order to lower the firing temperature, it has been attempted to add a glass frit to the dielectric powders. According to this method, the firing temperature can be lowered to about 1000.degree. C., but this is still higher than the melting point of silver (965.degree. C.) and furthermore, when the glass frit is added in a large amount, the proportion of the crystal phase decreases, and the dielectrics decreases, the dielectric characteristics deteriorate. Moreover, since the dielectric powders and especially the glass frit are produced by grinding, a particle size of less than 1 .mu.m can hardly be obtained. Therefore, it is difficult to attain a dense green body and thus it is difficult to obtain a film of thin thickness. That is, when a dielectric thin film is prepared by a tape casting method using dielectric powders obtained by the solid or liquid phase process, it is difficult to reduce the film thickness of the dielectrics to less than 20.mu. because of the large particle size of the resulting dielectric powders.
The electrostatic capacitance of capacitors has the following relation: ##EQU1## (wherein C indicates the electrostatic capacitance, S indicates the area, d indicates the distance between electrodes, .epsilon.o indicates the vacuum dielectric constant, .epsilon.r indicates the relative dielectric constant, and n indicates the number of the multilayers). That is, for making multilayer ceramic capacitors of small size and high capacitance, the number of layers should be increased since the area and the thickness have limitations. However, in the case of the abovementioned method in which the thickness of the dielectric layer is large, the attainable capacitance has limitation. As an attempt to solve this problem, it has been proposed to thin the dielectric layers. One layer of the multilayer capacitor has a thickness of about 20-40.mu. and if the thickness can be reduced to about 1-5.mu., a large capacitance can be obtained and moreover, the capacitor can be miniaturized. As the method for making a thinner dielectric layer, in addition to the tape casting method and printing method, there are vapor phase methods such as a sputtering method, vacuum evaporation method and CVD method. Furthermore, there are a sol-gel method and a method of producing dielectric thin films by coating and thermal decomposition of organometallic compounds. However, because of the thin thickness, shortcircuitting is apt to occur and leakage of current is large. It has also been known to use a ferroelectric glass-ceramics for obtaining a dense molded sheet.
Generally, the dielectric powders are produced as powders of 0.5-5 .mu.m by the solid state reaction conducted by firing the powders of oxides, carbonates or the like at high temperatures or by co-precipitation process comprising the reaction in an aqueous solution. In both of these reactions, the resulting powders comprise secondary particles formed by agglomeration of primary particles and there are problems in uniformity of the composition and crystallinity. In order to solve these problems, the hydrothermal synthesis has been proposed, but this process needs high temperatures and high pressures, resulting in increase of cost in view of cost of the equipment and productivity. As an alternative process, use of glass-ceramics is also proposed.
The process of using the glass-ceramics comprises blending the compounds in such a composition that a glass can be formed, melting the blend at a high temperature of about 1400.degree. C., quenching the melt to obtain a glass and then, heat treating the glass at 600.degree.-800.degree. C. to crystallize it. This process is not restricted by the size of the initial particles and dielectric thin films can be produced. Furthermore, due to its denseness, a low dielectric loss and a high dielectric strength can be realized. Fine single crystals can be obtained by dissolving a glass phase from this glass-ceramics. However, the composition capable of being vitrified is extremely limited and it is considerably difficult to increase the proportion of the crystal phase of high dielectric constant to more than 30%. Therefore, high dielectric constant cannot be obtained.